Telephones are widely used for business and private purposes. At the end of each telephone call, materials such as a document, an image, etc. related to a subject of the call may be transmitted to a call partner. Also in the middle of the call, the materials may be exchanged between the caller and the called person and they converse with each other while viewing the exchanged material. The materials are typically digital data. An electronic mail including such digital information is often directly transmitted from a call source to a call destination. In such a case, mail addresses need to be exchanged between the call source and the call destination. This operation is a troublesome. If the call source acquires the mail address of the call destination, there is a possibility that digital information unrelated to the call, such as junk mail, is transmitted to the mail address of the call destination.
The use of private information, such as an email address, other than the telephone, inconveniences the call source and the call destination. Japanese Laid-open Patent Publication Nos. 2007-166018 and 2000-174925 discuss methods of transmitting digital information to a call destination only in the middle of a call using a receiver such as a file server, a data communication device, or the like without using the private information such as a mail address. If the file server is used, a folder permitting the call source and the call destination to access thereto is generated in order to transmit the digital information. In the case of the data communication device, the call source and the call destination are connected to a data communication device operated during the call in order to transmit the digital information.